Flexural–torsional buckling initiates idiopathic scoliosis

Abstract

Initiation of the spinal deformity in idiopathic scoliosis (IS) has been attributed to an abnormal pattern of spinal growth during development. However, recent findings suggest that the earliest observable event in the pathogenesis of IS is a change in the shape of intervertebral discs with alterations in the shape of vertebrae being considered a secondary event. Starting from the previous description of the spinal deformity in IS as ‘buckling’ of the spine a new hypothesis describing the initial spinal deformity in IS as flexural–torsional buckling, a three-dimensional type of failure of axially loaded columns, is proposed. According to the new hypothesis the initiating event (the earliest observable event) in IS is flexural–torsional buckling developing from the flexible parts (intervertebral discs and ligaments) of the affected spinal motion segments. Since flexural–torsional buckling occurs in columns with a cross-section of one axis of symmetry characterised by a much greater in-plane than out-of-plane bending stiffness the new hypothesis predicts that the initiating condition (the condition promoting the initiation) of IS is ‘flexibility anisotropy’ namely significantly higher bending stiffness in lateral bending than bending stiffness in flexion–extension of a part of the spine. The parameter of ‘flexibility anisotropy’ as a factor for initiation of IS has never been suggested or tested before. The present hypothesis has implications in the research on the pathogenesis of IS as well as in the development of new methods for its treatment.